The present invention relates to a power supply apparatus for electric discharge machining. More particularly, this invention to a transistor type power supply apparatus for electric discharge machining, which can generate an intermittent pulse current using a semiconductor switching element.
Conventionally, there is known a power supply apparatus for an electric discharge machine which supplies an intermittent pulse current to a working distance formed between an electrode and a workpiece via a working fluid, and carries out electric discharge machining while controlling a relative position between the electrode and the workpiece by numerical control. The transistor type power supply apparatus for electric discharge machining is, for example, a representative of the above-mentioned power supply apparatus. This transistor type power supply apparatus for electric discharge machining generates an intermittent pulse current by a semiconductor switching element repeating an on-off operation.
This type of power supply apparatus for electric discharge machining will be described below with reference to FIG. 7(a) and FIG. 7(b). FIG. 7(a) shows a circuit configuration of the conventional power supply apparatus for electric discharge machining, and FIG. 7(b) shows a drive control system thereof.
The above power supply apparatus for electric discharge machining has a switching circuit for supplying a pulse current to a workpiece W and an electrode E. This switching circuit includes the first switching circuit 20 and the second switching circuit 30 connected parallel with each other.
The first switching circuit 20 is composed of the direct current voltage source V21, semiconductor switching elements S21, S22, S23 and S24 such as a FET or the like, and the current limiting resistor R21. On the other hand, the second switching circuit 30 is composed of the direct current voltage source V31, semiconductor switching elements S31 and S32, and diodes D31 and D32.
In FIG. 7(a), L21, L22, L31 and L32 denote a stray inductance of circuit, and C11 denotes a stray capacitance.
A drive control system of the power supply apparatus for electric discharge machining includes a discharge detecting circuit 31, an oscillation control circuit 32, a drive circuit 33 and a drive circuit 34. In this case, the drive circuit 33 drives and controls the semiconductor switching elements S21, S22, S23 and S24 of the above first switching circuit 20. On the other hand, the drive circuit 34 drives and controls the semiconductor switching elements S31 and S32 of the above second switching circuit 30.
Subsequently, operation of the power supply apparatus for electric discharge machining will be explained below. Assuming that a gap between the electrode E and the workpiece W (xe2x80x9cbetween the electrodesxe2x80x9d) is such that discharge or short-circuit does not occur, and when the switching elements S22 and S23 are turned off while the switching elements S21 and S24 are turning on, a voltage of the direct current voltage source V21 appears between the electrodes. Simultaneously, the stray capacitance C11 of the circuit is charged by the voltage of the direct current voltage source V21. A distance between the electrode E and the workpiece W is controlled by a numerical control device (not shown) and a servo drive control device so that a discharge is generated between the electrodes. When a discharge is generated by an output voltage of the direct current voltage source V21, first, a charge charged in the stray capacitance C11 of the circuit is discharged as capacitor to the inter-electrode, and thereby, a discharge start current Ic flows through there. By doing so, a conductive path is formed in the inter-electrode.
In order to maintain the conductive path thus formed, a current must be continuously supplied to the inter-electrode after the charge of the stray capacitance C11 of the circuit has been fully discharged; therefore, the switching elements S21 and S24 are kept as they are turned on.
From the direct current voltage source V21, a discharge holding current IR flows to the resistor R21, switching element S21, circuit inductance L21, workpiece W, electrode E, circuit inductance L22, switching element S24 and direct current voltage source V21 in succession, and thereby, the conductive path formed between the electrodes is maintained. In this case, the discharge holding current IR flows through the resistor R21; therefore, the maximum value of the discharge holding current IR is limited to IR (max)=V21/R21 by the resistor R21.
The discharge holding current IR is a relatively small current, and it is too weak for machining. Therefore, the discharge holding current IR has a function as pre-discharge current for supplying a large-current discharge machining current IS, which will be described latter.
Moreover, when turning off the switching elements S21 and S24 while turning on the switching elements S22 and S23, the above operation is carried out in a pattern of reversing a polarity of output voltage and current with respect to the gap between the electrodes.
The discharge holding current IR is a current appearing in between the electrodes at the same time with the generation of discharge. On the other hand, the large-current discharge machining current IS is supplied between the electrodes after the generation of discharge is detected. In this case, the large-current discharge machining current IS is output between the electrodes in a state of being delayed for a certain time from the first generation of discharge, as described latter.
The discharge detecting circuit 31 detects a drop of voltage between the electrodes (xe2x80x9cinter-electrode voltagexe2x80x9d) by the generation of discharge between the electrodes, and gives an instruction of large-current output to the oscillation control circuit 32. The oscillation control circuit 32 outputs a pulse signal having a time width set by a machining state between the electrodes to the drive circuit 34. The drive circuit 34 simultaneously drives on (turns on) the switching elements S31 and S32 only for the time width set in the oscillation control circuit 32.
When the switching elements S21, S24, S31 and S32 are all in an on state, a circuit is formed such that a plurality of direct current voltage sources having different voltage is connected. For this reason, there is a possibility of breaking down these elements of the circuit by a potential difference including a serge voltage. Thus, in the case of turning on the switching elements S31 and S32, the switching elements S21 and S24 are turned off as safety measures.
The switching elements S31 and S32 are simultaneously turned on, and thereby, from the direct current voltage source V31, the large-current discharge machining current IS flows to the switching element S31, circuit inductance L31, workpiece W, electrode E, circuit inductance L32, switching element S32 and direct current voltage source V31 in succession.
When no pulse signal is output from the oscillation control circuit 32, the drive circuit 34 drives off the switching elements S31 and S32. The discharge machining current IS continuously flows through the circuit by the induction of the circuit inductances L31 and L32; however, it is fed back and regenerated to the direct current voltage source V31 via the diode D32, circuit inductance L31, workpiece W, electrode E, circuit inductance L32, diode 31 and direct current voltage source V31.
FIG. 8 shows a waveform of discharge machining current obtained by the above operation in the conventional power supply apparatus and an output timing of each control signal. In FIG. 8, VWE denotes the inter-electrode voltage, and IC denotes a discharge start current by capacitor discharge of the stray capacitance C11 of circuit. Further, IR denotes a discharge holding current output from the first switching circuit 20, and IS denotes a discharge machining current output from the second switching circuit 30. Further, PK denotes a discharge detection output signal, PC denotes an oscillation control output signal, PD denotes a drive signal of semiconductor switching element, and IWE denotes an inter-electrode current.
The moment a discharge is generated between the electrodes, the discharge start current IC by the capacitor discharge of the stray capacitance C11 of the circuit appears between the electrodes. After the discharge is generated between the electrodes, the switching elements S21 and S24 of the first switching circuit 20 are being turned on, and therefore, the moment a conductive path is formed between the electrodes by the discharge start current IC, the discharge holding current IR starts to be output from the first switching circuit 20.
The discharge holding current IR is output between the electrodes via the inductances L21 and L22 of the circuit. Therefore, the discharge holding current IR does not rise instantaneously, and starts to flow at the gradient of V21/(L21+L22). In this case, the discharge holding current IR is limited by the resistor R21 as described before; for this reason, it does not reach the maximum value IR (max)=V21/R21 or more. The switching elements S21 and S24 are turned off until the switching elements S31 and S32 of the second switching circuit 30 are turned on; therefore, the discharge holding current IR has been output by that time.
On the other hand, the discharge is generated, and thereby, the inter-electrode voltage VWE drops to a discharge voltage Va; for this reason, the discharge detection circuit 31 detects the voltage drop, and then, outputs a discharge detection signal PK. However, in this case, a delay time is generated to detect the generation of discharge, and a time takes to output a signal; for this reason, the discharge detection signal PK is output after the time tk from the moment of the discharge is generated.
The oscillation control circuit 32 receives the discharge detection signal PK, and then, outputs an oscillation control signal PC. However, in this case, a delay time tc is generated likewise. Moreover, a delay time td is generated in an output signal PD of the drive circuit, and a delay time ts is generated in the switching element, likewise. Therefore, the discharge machining current IS appears between the electrodes after time tr (=tk+tc+td+ts) from the point of time t0 when the discharge is generated. The discharge machining current IS is a current output from the second switching circuit 30 via the inductances L31 and L32, and does not rise instantaneously as the discharge holding current IR. Further, the discharge machining current IS continues to increase at the gradient of V31/(L31+L32) for the duration of the switching elements S31 and S32 being turned on. Usually, the voltage of the direct current voltage source V31 is set about two to thee times higher than that of the direct current voltage source V21. Therefore, the gradient of the rise of the discharge machining current IS becomes steeper than that of the rise of the discharge holding current IR.
When the switching elements S31 and S32 are turned off, the discharge machining current. IS drops. The inter-electrode current IWE is a current having a relation of IWE=IC+IR+IS. The discharge holding current IR output from the first switching circuit 20 is supplied so as to supplement a time gap between the first discharge start current IC and the final large-current discharge machining current IS, and thereby, discharge machining is repeatedly carried out while maintaining a discharge state between the electrodes without interrupting the inter-electrode current IWE.
However, in the above conventional power supply apparatus for electric discharge machining, the upper limit value of the discharge holding current IR is limited by the resistor R21, and the current value is low in the initial state of transient state by the inductances L21 and L22 of the circuit. For this reason, the conductive path between the electrodes formed after the generation of discharge is not maintained. As a result, sometimes the supply of the discharge machining current IS fails. In particular, in a large-scale electric discharge machine, a distance between the power supply and the machine main body becomes long, and further, a feed cable for connecting between them inevitably becomes long. For this reason, the inductance of circuit becomes large, and there is the case where the discharge holding current IR does not rise after the discharge start current IC disappears; as a result, the conductive path formed between the electrodes is interrupted.
Further, in the resistor R21, there exists an inductance component by resistance windings, and in the case where the inductance of resistor becomes inevitably large in order to obtain a necessary resistance value, there is an influence of further disturbing the rise of discharge holding current IR.
Furthermore, the first discharge start current IC is a current by capacitor discharge, and in fact, includes an oscillating component. For this reason, even if the maximum value of the discharge holding current IR is previously set slightly larger, the discharge holding current IR is offset by a negative component of the oscillation; as a result, the conductive path formed between the electrodes is interrupted.
As described above, when the conductive path between the electrodes secured by the discharge start current IC is interrupted before the discharge machining current IS is supplied, it is impossible to obtain the operation of stably supplying the discharge machining current IS between the electrodes by a pre-discharge current. As a result, various faults are generated in discharge machining.
In a state that the conductive path formed between the electrodes is interrupted, the output of the second switching circuit power supply apparatus 30 is an open state; for this reason, no discharge machining current IS flows, and in this case, normal discharge machining is not carried out. When the above state frequently occurs, the number of effective discharge times is reduced; as a result, a problem arises such that a machining (working) speed to be inherently obtained is not obtained, and it is impossible to further improve the above machining speed.
In order to output a large current for a short time, the voltage of the direct current voltage source V31 is usually set about two to three times higher than that of the direct current voltage source V21. However, in the case where no conductive path is formed between the electrodes and the circuit is an open state, a high voltage of the direct current voltage source V31 is applied between the electrodes. In other words, a discharge is newly generated by the high voltage, and thereafter, a large current is suddenly applied between the electrodes without generating a pre-discharge. For this reason, in the case where the electrode E is a thin electrode such as a wire electrode, the wire electrode is disconnected, and further, a machining surface becomes coarse even if no disconnection is generated in the electrode, and therefore, this is a factor of deteriorating a machining accuracy. As a result, a problem arise such that a stable discharge machining characteristic is not obtained.
The above problem has been pointed out in a power supply apparatus for wire cut discharge machine disclosed in Japanese Patent Application Publication No. 5-9209. According to this publication, a circuit having inductance and capacitor connected in series is arranged in parallel with the gap between the electrodes, and thereby, a conductive path between the electrodes after the generation of discharge is maintained, and the discharge state is stably continued so as to prevent a reduction of machining efficiency.
However, in this case, the extra capacitor must be inevitably added between the electrodes. For this reason, an electric capacitance on the voltage source side increases together with a stray capacitance of circuit. As a result, a rise time constant becomes large when the output voltage is applied between the electrodes, and therefore, the rise of inter-electrode voltage is delayed.
Accordingly, a voltage application time until the discharge is generated becomes long, and the number of effective discharge times is reduced; for this reason, a problem arises such that a machining efficiency is not sufficiently improved.
Moreover, by the value of the added inductance and capacitor, a natural vibration (oscillation) frequency is obtained. In recent years, a bipolar type power supply apparatus for electric discharge machining has been mainly used. In this type power supply apparatus, a polarity of voltage applied between the electrodes is alternately replaced, and then, oscillation output is made. In this case, the added capacitor repeats a charge and discharge operation by at least oscillation frequency of voltage application. Further, in a capacitor used for high frequency, an induction loss exists; for this reason, not only the oscillation frequency is limited, but also heat is generated by the induction loss. As a result, a problem arises such that supply energy loss is generated.
Therefore, an object of the present invention is to provide a power supply apparatus for electric discharge machining, which can stably maintain a conductive path formed between the electrodes without a disappearance of the formed path for the duration of delay time until a machining current is supplied from a pre-discharge in discharge machining, and can improve a discharge machining efficiency and quality without a fail of supply of the discharge machining current and unnecessary damage to an electrode and a workpiece.
The present invention provides a power supply apparatus for electric discharge machining, which includes first and second switching circuits connected in parallel with each other, supplies a pulse current to an inter-electrode distance an electrode and a workpiece from the first switching circuit, and subsequently, from the second switching circuit, and carries out discharge machining while controlling a relative position between the electrode and the workpiece, characterized in that the apparatus has a current loop including: a diode, which is supplied with a forward current at the same time a voltage for generating a discharge is output or before it; and a resistor, and blocks a supply of forward current to the diode the moment a discharge is generated, and further, outputs a reverse recovery current of the diode to in the gap between the electrode and the workpiece.
Therefore, the moment when a discharge is generated, the reverse recovery current of diode is output between the electrodes and the workpiece prior to the machining current output by the second switching circuit. By doing so, for the duration of the delay time from the pre-discharge to the supply of machining current in discharge machining, it is possible to stably maintain the conductive path formed between the electrodes without extinguishing the conductive path.
The power supply apparatus for electric discharge machining according to next invention further includes a semiconductor switching element for blocking the supply of forward current to the diode. Therefore, by the on-off control of the semiconductor switching element, the supply of forward current to the diode is blocked, and then, the moment when a discharge is generated, the reverse recovery current of diode is output in the gap between the electrode and the workpiece prior to the machining current output by the second switching circuit. By doing so, for the duration of the delay time from the pre-discharge to the supply of machining current in discharge machining, it is possible to stably maintain the conductive path formed between the electrodes without extinguishing the conductive path.
In the power supply apparatus for electric discharge machining according to next invention, there is provided a circuit configuration such that a reverse voltage is applied to both terminals of the diode the moment when a discharge is generated, and outputs a reverse recovery current of diode generated at that time in the gap between the electrode and the workpiece. Therefore, the moment when a discharge is generated, a reverse voltage is applied to both terminals of the diode, and the reverse recovery current of diode is output in the gap between the electrode and the workpiece prior to the machining current output by the second switching circuit, the moment when a discharge is generated. By doing so, for the duration of the delay time from the pre-discharge to the supply of machining current in discharge machining, it is possible to stably maintain the conductive path formed between the electrodes without extinguishing the conductive path.
Further, the present invention provides the power supply apparatus for electric discharge machining, characterized in that a plurality of diodes is connected in parallel or in series. Therefore, the number of diodes connected in parallel or in series is set to a proper value in accordance with a required current value of reverse recovery current.
Further, the present invention provides the power supply apparatus for electric discharge machining, characterized in that another direct current voltage source is provided as a direct current voltage source for supplying a forward current to the diode, except for the direct current voltage source constituting the first or second switching circuit. Therefore, a forward current is supplied to the diode by another direct current voltage source except for the direct current voltage source constituting the first or second switching circuit.
Further, the present invention provides the power supply apparatus for electric discharge machining, characterized in that the forward current of the diode is supplied from the direct current voltage source of the first switching circuit. Therefore, a forward current is supplied to the diode by the direct current voltage source constituting the first switching circuit.
Further, the present invention provides a power supply apparatus for electric discharge machining, which includes first and second switching circuits connected in parallel with each other, supplies a pulse current in the gap between an electrode and a workpiece from the first switching circuit, and subsequently, from the second switching circuit, and carries out discharge machining while controlling a relative position between the electrode and the workpiece, characterized in that the apparatus has a current loop including: a capacitor, which charges the capacitor at the same time a voltage for generating a discharge is output or before it; and a resistor, and outputs a discharge current from the capacitor in a gap between the electrode and the workpiece prior to a machining current output by the second switching circuit after a discharge is generated.
Therefore, after a discharge is generated, a discharge current from the capacitor is output in the gap between the electrode and the workpiece prior to the machining current output by the second switching circuit. By doing so, for the duration of the delay time from the pre-discharge to the supply of machining current in discharge machining, it is possible to stably maintain the conductive path formed between the electrodes without extinguishing the conductive path.